In a multi-cylinder engine, an intake manifold is attached to a cylinder head of the engine. In addition, there is one type of engine having the intake manifold and an air cleaner integrally formed as an unitary structure. In this latter arrangement, as illustrated in FIG. 11, an intake manifold 102 and a lower case 106 of an air cleaner 104 are formed integrally as one piece. An upper case 108 is fitted onto the top of the lower case 106, thereby forming the air cleaner. A cleaner element (i.e., a filter) 110 is disposed between the cases 106 and 108 for partitioning the air cleaner 104 into a dust chamber 112 and a clean chamber 114. The dust chamber 112 is formed on the side of the lower case 106, while the clean chamber 114 is defined on the side of the upper case 108. A first plate-like member 118-1 projects up to an intermediate height and extends between first and second branch pipes 116-1 and 116-2 of the intake manifold 102 positioned in the dust chamber 112. A second plate-like member 118-2 also projects up to an intermediate height position and extends between the third and fourth branch pipes 116-3 and 116-4 of the intake manifold. A third plate-like member 118-3 projects up to an intermediate height position and extends between the second and third branch pipes 116-2 and 116-3.
In the above-described conventional engine intake system-draining structures, saddles or valleys, like passes in a mountain range, are formed between the adjacent branch pipes of the intake manifold. The saddles form part of the bottom of the air cleaner. As a result, the bottom of the air cleaner is dramatically serrated or sinusoidal in shape, thereby producing the saddles or valleys between the branch pipes of the intake manifold. This causes an inconvenience in that it is difficult to establish a drain hole in order to permit resident water in the saddles to be efficiently removed.
Further, when a plurality of water drain holes are defined at the bottom of the air cleaner so as to correspond with the serrated configurations of the bottom, this results in a complicated structure which is difficult to manufacture and is expensive. This is disadvantageous from an economical viewpoint.